Cross training is a great way to increase your fitness while allowing your body to recover. If all you do is run, you will most likely eventually develop an injury. Cross training not only increases your overall fitness, it also helps prevent injuries. Strive to add in cross training two or more days per week.
Cross training machines provide low impact cardio and strength benefits.
Strength training is important for any fitness regiment. If you don't belong to a gym, there are many ways to strength train at home. Don't lift on run days.
If you live in northern climates, snow showing, skating and cross county skiing provide low impact fitness and gets you outside.
Biking is one of the best ways to cross train. Your legs will get stronger and you'll gain cardio fitness. Wear your helmet.
Swimming is great exercise. The "I Used To Hate Running Plan" works for swimming. It is easy to increase your swimming distance just by swimming consistently.
Yoga helps you increase the strength and elasticity of your muscles. It is a great cross training activity.